Looking for input on the balance of this, the first 3 powers I like and are just monkey-wrenches in the investigators plans, the thing I'm unsure about is the reversal of fortunes. My thinking was that Arkham is rarely, if a game is being played responsibly, overrun. This would add a real chance of that occuring, and if the Ancient One was about to awaken, you might gain a few rounds from a favourable Terror/Doom track swap, but you'd now have to deal with keeping monsters in check to prevent the monster cap x 2 insta-awaken from happening.
Dealing with the Cultist would be no problem, a quick gunshot in the night and he would be on his way. The cultist's robes fell to the ground and a snarling Hound of Tindalos faced him down instead.
Whenever you fight a Cultist, return the Cultist to the cup and draw another monster and resolve combat with that monster (even if that monster is another cultist).
Wait, Where am I?
He stood in the middle of the Woods, the visions of sterile corridors fading quickly. This wasn't St-Mary's and those weren't doctors, he looked for a place to hide as the Sheldon gang approached.
Whenever you draw a location card, roll a D6, on 1-2 resolve the first location's text, on 3-4 resolve the second, on a 5-6 resolve the third. If the location rolled is not the location you are at, move to that location immediately. If there is an open gate on that location, you are drawn through instead of having an encounter (but are not delayed).
She stared at him in disbelief, her studies at Miskatonic led her to believe he was speaking ancient Sumerian, but she was far from fluent.
When trading items, the number of cards and type that will be traded must be agreed on (1 for 2, 1 for none, etc). Once the exchange is agreed upon, both players make a Lore check. If both succeed, the exchange happens. In the case of a failure, the player must shuffle all his cards belonging to the card type that was to be exchanged (if only a common card was to be exchanged, only shuffle your common items, if it was a mix, shuffle the mix). The other player must draw the items to be exchanged randomly from the shuffled pile. Note: If only one player fails the Lore check, this only effects one player, the other player must proceed with the trade normally (exchanging the agreed upon cards).
Reversal of Fortune:
The Great Old One stirred fitfully, thrashed against the bonds that kept him from our world, unleashing terror into the streets of Arkham. When he was done, strangely, he slept even more soundly than before.
Whenever a Gate Burst is drawn, the current values on the Doom and Terror track are reversed (even if this means a terror level of above 10). Keep a separate discard pile of the Allies discarded in case the tracks are reversed again.
Also, looking for a name of the Herald, so far I'm thinking Enki from Babylonian mythos (namely from the confusion and muddled languages stuff) but would be open to something more anchored in Lovecraftian Lore.
Alright, the first thing I would recommend is shoving it onto a Herald sheet in Strange Eons to see how all the text fits, because you have a ton of it. In order for this to be readable on a Herald sheet, you might need to cut down on a lot of words or possibly remove a lot of flavor text.
So I assume that you don't swap the cultist out until you've officially engaged it in combat, which means if you try to sneak past the cultist it means a cultist for the first pre-combat evade check. The ability itself is fine, but it really hoses investigators that make a living sneaking past monsters.
Wait, Where am I?:
It's okay to just say "Roll a die", since the Arkham standard die is a D6 already. The only weird issues are with Silver Twilight Lodge / Inner Sanctum, and one case up in Innsmouth with Innsmouth Jail / Jail Cell. As it stands, its possible for people to have an Inner Sanctum encounter without a Silver Twilight Lodge membership, and there technically isn't a location called Inner Sanctum to move to (although I guess most people would just move to STL anyway). As for the Jail Cell, it basically says that any investigator that ends in Esoteric Order or Innsmouth Jail can wind up immediately in the Jail Cell, and people in the Jail Cell might be denied encounters that would normally free them (because they have Esoteric Order or Innsmouth Jail encounters instead, sometimes). I would probably just add "If there are only two locations, roll 1-3 and 4-6 instead."
Okay, that's some really roundabout phrasing sir. With the way you phrased the ability, I understand why it ended up that way, but maybe that's a sign that you should change the text for the sake of simplicity. Might I recommend the following change: "Whenever two investigators attempt a trade, they declare the items they are trading and each make a Lore (+0) check. For each player that fails the Lore check, they instead choose a number of new random items they own equal to the number of items they would've traded, and trade those items instead. Nothing happens to investigators that pass the Lore check. Investigators can only initiate one trade per turn."
Reversal of Fortune:
I'm afraid this ability breaks the Arkham rules, sir. The Terror Level is a value that cannot be lowered by any means, merely prevented from raising. Because of the nature of the ability, I don't think you can simply rephrase this one. You probably just need to remove this ability entirely. Sorry.
Overall, the effects are all thematically linked, but not very difficult. Probably the worst part of it is the sheer amount of unpredictability, but that certainly isn't going to stop players (Arkham is all about unpredictability). The hardest clause is definitely the cultist swapping just because they are normally hard to evade but piss easy to fight, and now they are hard to evade and ??? to fight. People wouldn't be especially bothered by the location swapping in most cases, and occasionally it may create shortcuts for people to enter gates that are otherwise difficult to enter. As far as the trading goes, here's how most cases will play out: I attempt to trade with someone, somebody fails a lore check, things go wrong………….just immediately initiate another trade……repeat process until everyone gets what they want eventually (doesn't even stop people with no Lore). So, overall not too difficult and some of the abilities need to be reworded.
As far as names go, Enki is probably fine. If you want something original, how about "Vitiatus, Overbeing of MIsdirection"